A Michelson interferometer in its basic form comprises a beam splitter and two end mirrors, one in each optical path created by the beam splitter. To this basic arrangement is often added a compensator of the same material, thickness, and angle of incidence as the beam splitter substrate. This balances the optical path length in both legs at all wavelengths. A complete spectrometer based on a Michelson interferometer generally includes a light source, a means of limiting the angular subtense of light traversing the interferometer, a means of placing a sample to be tested in the optical path, and a means of detecting the light after it has traversed the two legs of the interferometer and recombined. It also contains some means of varying the optical path length difference (OPD) between the two interferometer legs to produce an interferogram, and a means of measuring this OPD, often with a position encoder based on an auxiliary monochromatic light source. Since the advent of fast Fourier transform algorithms in conjunction with a digital computer the Michelson interferometer and numerous variants of it have been used to measure the spectrum of light sources, either directly or after passing through a material with properties that can be determined by the measurement of spectral absorbance. Several authors have provided detailed reviews of this type of spectrometer and its merits relative to other spectrometers for chemometric measurements. See, e.g., Griffiths and De Haseth, Fourier Transform Infrared Spectroscopy, Wiley Interscience, 1986.
A simple Michelson interferometer with flat end mirrors has several problems and limitations. A Fourier transform spectrometer application typically requires that the OPD be scanned, which can be difficult with current interferometers. Conventionally, one of the end mirrors is translated along the optical axis while keeping the surface strictly perpendicular to the optical axis and constant to within a small fraction of the wavelength of the light being measured. In the long wave infrared region the requirement can be met relatively easily with a mechanical translator but it becomes increasingly difficult at shorter wavelengths. This mechanical stability problem is found to be a critical factor in chemometric applications in which the quantity of an analyte of low concentration and weak spectral features must be determined. In many such applications, high spectral resolution is of secondary importance to such factors as high throughput and spectral response stability with temperature and other environmental perturbations.
Various methods have been tried to alleviate the problem of maintaining alignment while changing OPD. One method involves the use of two retroreflectors instead of the two flat mirrors. See, e.g., Griffiths, pp. 143-147. A retroreflector changes the direction of the light by exactly 180 degrees but not necessarily along the same axis. The use of retroreflectors eliminates the tilt problem (maintenance of perpendicularity to the optical axis) but introduces a problem with shear (maintenance of colinearity of optical axes from the two interferometer branches). An interferometer with either a tilt or shear misalignment will produce a smaller useful signal for a given input light intensity and angular subtense than one in which these errors are not present. Furthermore, the effect of these errors will be wavelength dependent. Generally, conditions are such that the allowable tolerance on shear is much easier to meet than the tolerance on tilt. However, for large throughput designs, involving an extended source, the shear tolerance can still be quite difficult to adjust and maintain in practice, requiring that the retroreflector lateral position be maintained within less than a few wavelengths of the light being measured while it is moved along the optical axis to change the OPD. A retroreflector is constructed from two or more reflective surfaces that must be adjusted and maintained in precise alignment to provide a wavefront quality that does not change by more than a small fraction of a wavelength. Although at least two retroreflector designs, the cube corner and the cat's eye, are in common use, they require precision construction much more difficult to produce than a simple flat mirror.
Another related method is that of Steel (1970). Steel, pp. 48-49 In this method, two flat end mirrors are fixed and precisely aligned. A retroreflector is then added in a double pass arrangement to direct the light onto one of the end mirrors and then back through the retroreflector on the return trip. The advantage of this technique is that the retroreflector can be moved to create the OPD without generating tilt or shear errors, allowing a simple, non-precision mechanism to move the retroreflector. Disadvantages include the requirement for a high quality retroreflector with an aperture size at least twice as large as the beam going through the interferometer. The arrangement also does not preclude the need to adjust the angle of one of the end mirrors to eliminate tilt error. The path length through the system is also substantially increased over that of the basic Michelson, making it more difficult to collect all the light from an extended source. The double pass nature of this design also increases its sensitivity to externally induced vibration.
A second category of interferometer OPD scanning methods is based on the movement of a refractive element or elements. One example method, shown by Steel (1970), uses the movement of a wedged refractive plate into the beam in varying amounts. Steel, p. 49 A fixed plate, wedged at the same angle but in the opposite direction serves to compensate for tilt errors that would otherwise arise as a function of wavelength. Although affording considerable reduction in sensitivity to tilt of the moving component, tilt error is still introduced with tilt in the wedged plate if flat end mirrors are used. This problem is reduced but not eliminated in U.S. Pat. No. 4,165,938 (Doyle, 1979), where linear motion of a wedged refracting prism is used along with retroreflectors replacing the flat end mirrors. In this arrangement tilt error is avoided by the use of retroreflectors but shear error is still introduced by tilting the prism, especially for off-axis rays. Another embodiment of the refractive scanning technique is shown in U.S. Pat. No. 3,482,919 (Barringer, 1969). In this patent the compensator plate of a basic Michelson interferometer is rotated to generate the OPD. This method is based on the principle that the optical path length through a window material can be changed by rotating the window about any axis not normal to the window surface. If the two sides of the window are parallel, the OPD is changed without changing the direction of a ray, and thus no tilt error is introduced regardless of the plate position or rotation angle. This allows the OPD to be varied without disturbing the alignment of fixed, critically aligned components, such as the beam splitter and end mirrors. The penalty paid for this simplicity is that the OPD is not a linear function of the rotation angle. Various performance degradations can arise from this, which will be discussed in detail below. U.S. Pat. No. 4,654,530 (Dybwad, 1987) deals with the linearization issue by passing both legs through the same parallel plate but in different directions, so that as the plate is rotated, the OPD increases in one leg while decreasing in the other. It is then found that the net OPD change is much more linear as a function of the rotation angle. The resulting interferogram is also more symmetrical about the center burst. This is all accomplished at the expense of two additional flat mirrors and a substantial increase in total path length. In U.S. Pat. No. 4,872,756, (Hill, 1989) the same thing is achieved with the addition of only one mirror to the basic Michelson arrangement. This patent also shows an arrangement in which two plane parallel plates are added to the basic Michelson interferometer, one in each leg. By rotating these two plates together the same linearization of the OPD-angle relationship can be accomplished as with the single plate shared by both legs. The refractively scanned interferometer also enjoys a much better immunity to externally induced vibration since it can be made to require much larger physical movement of the scanning element to achieve a given OPD than does the interferometer based on the movement of a mirror. This makes it an ideal candidate for portable use outside the laboratory. A problem that often exists with refractive scanned interferometers is that the allowable extended source size in one direction is smaller than that for a Michelson interferometer and therefore the aperture size must be larger for a given throughput.
Alignment maintenance is another challenging problem with interferometers. Various “self-compensating” designs have been used which involve a number of flat mirrors or mirrors in conjunction with refractive elements. In these designs, the optical arrangement is such that the precision required for maintaining the optical alignment is built separately into each piece or sub assembly and not on the relationship between subassemblies. For an example, see U.S. Pat. No. 6,504,614 (Messerschmidt and Abbink), in which the required precision is contained within the parallelism of two faces of two solid refractive components. In European Patent no. 0 681 166 B1 (Turner, 1995) the critical precision is built into two subassemblies consisting of flat components with bonded spacers to keep the subassembly components precisely parallel. One shortcoming common to these designs is that the optical path length through the instrument becomes larger than through the simple Michelson interferometer, often by a rather large factor. The result is that, for an extended source, excessive vignetting cannot be avoided unless the clear apertures are made larger than they would need to be with an interferometer with short optical path length.